Email is a reasonably secure way to communicate, but only as long as both sides have encrypted connections to their email servers. A growing threat to this is Internet connections that for some reason or another fail to create (or actively prevent) encryption of private data.
Google, working with the University of Michigan and the University of Illinois, has developed methods to identify when an email originates from just such a connection — and Gmail will now warn users when that's the case. If encryption was being interfered with, an email may have been altered or loaded with malware disguised as attachments from a known correspondent. These upcoming warnings should prevent that.
Another threat identified by the team is shady DNS servers, the systems that direct your browser to the correct IP address to connect to websites and services. These malicious servers serve up fake data when users are looking to get to Gmail, possibly allowing tampering with mail traffic. Fortunately, this is a relatively rare case, Google wrote in the blog post discussing these security issues.
The warnings for emails originating from bad connections should start appearing in the next few months.